I recall that there was a get-together of climate scientists earlier in the year before the late 2009 Copenhagen confab by politicians. It turns out that this "international congress" of climate scientists took place in early March of 2009, if I'm reading the news right. (The Copenhagen policy maker meeting would subsequently take place in December of 2009.) I believe the purpose of the climate scientists meeting earlier was to provide a background of informed advice for the later political meeting of Obama and others, late in 2009. (Even then, many were already saying that the political meeting would be a failure with no meaningful agreements to come out of it -- and some scientists were also publicly saying early in 2009 that the chances of achieving the 2℃ target were all but gone already, anyway.)

Part of this earlier climate scientist meetup was to evaluate the most recent information that had been added since the 2007 IPCC report, which itself by the time it came out was already becoming dated. I believe the number 2℃ probably had been floated beforehand, too, but this group of climate scientists also specified that particular number as important.

Chances are, the 2℃ number has been discussed on any number of occasions as a reasonable one to "accept" as a boundary. Probably well before 2009. But I think the political agreement shaped in December of 2009 in Copenhagen got their number from the most recent report developed by climate scientists earlier that year.

See the first link below.

EDIT: I believe most climate scientists are pretty na?ve about manufacturing the consent of populations or about politics and power. Some may be wiser than others, of course. Their statement about the 2℃ limit, which comes from the link I already provided, says, "temperature rises above 2℃ will be very difficult for countries to cope with." I think that is a conclusion made by some scientists given that, "recent observations show that societies are highly vulnerable to even modest levels of climate change." I now recall reading back then that anything over 3℃ would eventually collapse the Amazon rainforest, lead to serious water shortages in South America and Australia, and lead to a near-extinction of tropical coral reefs. That may still be the opinion of many climate scientists today, too. But I don't know since I haven't read anything since then on that topic.

They weren't stepping too far off the reservation regarding the 2℃. They couldn't select 3℃, because the implications were atrocious. A fractional figure wouldn't work -- just causes people to ask "why 2.4℃ instead of 2.3℃ or 2.5℃?" Which completely derails the discussion. So 2℃ it is. In any case, if it is true that societies are very vulnerable to modest levels of change, and if the scientists felt that 2℃ is more than a "modest level" and that 3℃ was untenable, then it logically follows directly that they should recommend 2℃. Nothing in their statement says anything about how difficult it may be, politically. However, Dr. Kevin Anderson and Dr. Trevor Davies were both outspoken at the time. You can read a blurb about what they said in the 2nd link below.

"Is it possible that scientists consider this a political limit while politicians consider this a scientific limit?"

I know you love pushing the possibility that science is politically motivated. What I've heard, though, is a scientific outlook that says " We can make a pretty good guess at what happens to the Earth within a 2C rise, above that it gets considerably less predictable." Most scientists I've known would add "and consequently considerably scarier".

The 2 degree C limit may come from climate models as a point at which methane from tundra and carbon dioxide outgassing from the oceans lead to dramatic additional warming.

I agree that models are not perfect. Unlike what some "skeptics" claim, I do not believe in global warming BECAUSE of these models. I do not need a computer to tell me that adding a greenhouse gas to the atmosphere causes warming. Of course it does.

Until computer models are actually perfect, or until Earth actually does warm by 2 degrees C, we may never know what will happen then. But, from what we do know, we should hope for one of two things;

1. That we keep the increase of temperature to below 2 degrees C, and

2. That whatever we keep the temperature to, higher or lower than 2 degrees C, that we do not trigger these feedbacks.

The IPCC thinks that a carbon doubling would raise Earth's temperature 3 C. http://en.wikipedia.org/wiki/Climate_sen...

During the first decade of this century, atmospheric CO2 concentration increased 5.6% http://www.woodfortrees.org/plot/esrl-co...

Thus, extrapolating that rate of increase to the entire century:

(10 decades)(ln[1.056])/ln[2] = 0.79 = 79% http://en.wikipedia.org/wiki/Beer%E2%80%...

so 79% of the carbon sensitivity would be realized at that rate.

79% of 3C = 2.4C

Now make it a round number: 2C

Rounding up would not require any international legislation. People would laugh at a threat of 1C. They do laugh at 2C as it is. So 2C it is.

Edit @Climate Realist: <>

Using that logic, adding an anti-greenhouse gas like O3 to the atmosphere would "of course" cause cooling. However, the IPCC disagrees (1), and with good reason. O3 absorbs UV light efficiently for wavelengths below 300 nm. It absorbs a lot of incoming radiation that way. O3 absorbs infrared (out going radiation) much less efficiently at 10 microns with obvious effect. The amount of light absorbed add O3 added to the atmosphere is calculated:

A = 100% - e^(-kc)

A = absorbed light in an absorption band

e = 2.7

k = a constant for the particular absorption band that depends on the units used for concentration.

c = the concentration of the gas doing the absorbing.

taking the derivative:

dA/dc = ke^(-kc)

Since k depends on the units of concentration, I pick a unit of concentration such that:

for the UV band: k=2, and for the 10 micron band: k=1. Let us pretend that the intensity in both bands are the same for the sake of argument.

The anti-greenhouse effect will become a greenhouse effect for addtional O3 when

1e^(-c) = 2e^(-2c)

2 = e^c

c = 0.69 in some specially select unit. This number would change if the intensity were accounted for, but still exist. Thus, whether a gas cools or heats Earth depends on its location in the atmosphere, the effects of other gases and particles, and its concentration:

CO2:

Greenhouse (Main peak) = 15 micron

Anti-Greenhouse = 2 micron

CH4:

Greenhouse (Main peak) = 8 micron

Anti-Greenhouse = 2.5 micron (2)

I asked my students to calculate the greenhouse effect of CO2 at its current concentration. They came up with about 1 Kelvin per doubling of CO2. However, that calculation did not include anything about anti-greenhouse effects from the 2 micron band or feedbacks. There is evidence that the net effect of adding more CO2 to the atmosphere at current concentrations does not have a significant effect on the energy balance of Earth (3). This is not an "of course" thing that requires no computer at all.

Edit @Ottawa Mike: <<... scientists consider this a political limit while politicians consider this a scientific limit?>>

The Stern Review, <> -- http://en.wikipedia.org/wiki/Stern_Revie...

It is the source that is mostly used as a basis for the effects of global warming (6). However, it has been poorly recieved by many scientists due to its obvious political nature, and its overly alarmist nature (4). Some scientists do consider that the assumptions in the report were political in nature (7), but I doubt that the UN politicos that set climate policy know enough about science to be aware of that (5).

I believe it came out of AR3 (2001) where possible outcomes were grouped with 0-2 degrees a "small" increase and 2-3 degrees a "medium" increase. Chapter 19 of that report is largely about possible effects at different levels of increases.

http://www.grida.no/publications/other/i...

In a real sense, it doesn't matter because we are doing almost nothing to try to stay within that "limit."

Just read an article today: "A pledge governments made in 2009 to avoid global warming above 2°C is unlikely to be abandoned before the 2015 climate treaty is signed..." http://www.rtcc.org/governments-unlikely-to-ditch-engrained-2c-goal/

There's nothing particular about that story, it and variations on it appear all the time. I'm trying to find the origin of the 2C figure.

As the above story alludes, the 2009 reference is to the Copenhagen climate meeting in November of 2009. The BBC reported back then: "The language in the text shows that 2C is not a formal target, just that the group "recognises the scientific view that" the temperature increase should be held below this figure." http://news.bbc.co.uk/2/hi/8422307.stm

So my real question is: What is the origin of the "scientific view" of the 2C limit which was inserted into the text of the Copenhagen Accordt and obviously is still part of the AGW lexicon today?